According to VMware's RCLI Installation and Reference Guide, the data center and resource pool parameters to vmware-cmd -s register are optional, but nonetheless, it kept telling me "Must specify resource pool".
Eventually, I found a forum thread that contained the answer: Resources. (Kinda like "plastics" only different.) I don't have a data center set up, but I found that using almost any word for the data center name seemed to work.
So, in the end, this is what worked for me (your milage may vary):
vmware-cmd [conn options] -s register '[datastore-name] vm-dir/conf.vmx' root Resources